Epidemiological and laboratory evidence indicates that environmental contaminants such as TCDD affect vertebrate heart development. The receptor for TCDD is the Aryl Hydrocarbon Receptor (AHR), a ligand-activated transcriptional activator. We have used zebrafish, an ideal system for studying heart development in vivo to document the cardiotoxic response to TCDD. These results support a model in which TCDD disrupts normal gene regulation in the developing heart. Our microarray experiments have defined the pattern of gene expression changes produced by TCDD in zebrafish embryonic and adult hearts. We find that AHR activation regulates a different set of genes in embryonic heart cells than in juvenile heart cells. Adult hearts are resistant to TCDD cardiotoxicity, except when regenerating lost tissue after surgical wounding. The sensitivity to TCDD in the embryonic and regenerating adult heart correlate with periods in which epicardial cell migration, division, and differentiation are critical to the heart. This suggests that TCDD may be affecting epicardial cell function. We have developed methods for transplantation of cells from embryonic zebrafish hearts into the wounded adult muscle. These embryonic cells become incorporated into the wounded heart muscle. This project is focused on the following questions: 7 Are the cardiotoxic effects of TCDD mediated by AHR activation in the heart? This will be addressed by using transgenic zebrafish to manipulate AHR activity specifically in the heart. 7 Does TCDD alter epicardial function in the embryonic and regenerating adult heart? This will be addressed by determining whether TCDD blocks epicardial cell formation and migration in the embryonic heart. In addition, we will determine whether TCDD alters the activation, migration, growth, and differentiation of embryonic-like epicardial cells in the adult heart in response to wounding. We will compare the genes affected by TCDD in adult wounded hearts to those affected by TCDD in embryonic hearts. Finally, we will determine whether TCDD prevents transplantation of embryonic heart cells into wounded zebrafish myocardium. 7 Why does TCDD activation of AHR affect a different set of genes in the embryonic heart than in the juvenile heart or the developing jaw? We will test the hypothesis that the AHR complex is different in different tissues, producing different DNA binding site specificity in different tissues. As an alternative, we will test the hypothesis that the chromatin structure at a given gene may allow AHR binding in the embryonic heart, but in other tissues the chromatin structure at this gene would be different, preventing AHR binding. This will be tested using ChIP assays coupled to microarrays (ChIP-chip assays).